This invention relates specifically to stents but also generally to any expandable, implantable endoprosthesis such as grafts, stent-grafts and the like. When the term ‘stent’ is used herein, it is to be understood in a general sense as including all such expandable prostheses, unless otherwise indicated.
The use of endoprostheses such as stents, stent-grafts and grafts is well known in maintaining the patency of bodily vessels including blood vessels and biliary ducts. Typically, an endoprosthesis is implanted in a vessel which has been occluded, which is subject to an aneurysm, which has a lesion present or is otherwise damaged. Often, during the implantation of the endoprosthesis, the vessel will suffer from trauma. The trauma may be as a result of the dilation prior to the implantation of the endoprosthesis, the presence of a foreign body (the endoprosthesis) in the bodily vessel or as a result of other causes. Regardless of the source of the trauma, the vessel may be in a weakened and inflamed state as a result of implantation of the endoprosthesis. Although it is desirable to maintain the vessel at as large a diameter as possible to minimize the possibility of restenosis, the weakening of the vessel resulting from the trauma limits the extent to which the vessel can be dilated.
The endoprostheses that are currently available are typically balloon expandable, self-expanding, or balloon assisted self-expanding devices. Balloon expandable stents achieve their maximum diameter upon expansion by a balloon. Once they have been seated in the vessel, they are incapable of further expansion unless a balloon is reinserted in the stent and expanded. Self-expanding and balloon assisted expandable stents, on the other hand, continually exert an outward force as they try to attain their maximum possible diameter. Thus, even after the stent is implanted, if it has not reached its maximum diameter it continues to try to open further exerting a force on the vessel walls.
It would be desirable to provide an endoprosthesis which has some of the characteristics of balloon expandable stents following deployment and which has some of the properties of self-expanding stents after a predetermined period of time or after the application of a predetermined amount of force thereto. In particular, it is desirable to provide an endoprosthesis which does not impart to the vessel walls the outward forces associated with a self-expanding stent while the vessel is recovering from the trauma of the deployment procedure and yet provides the outward expanding force of a self-expanding stent when the vessel is sufficiently recovered from the trauma.
It is also desirable to provide an endoprosthesis which only partially expands or does not expand at all during deployment and which expands at some later point following deployment.
The present invention provides such an endoprosthesis.
For the purposes of this disclosure, unless otherwise indicated, the term ‘degradation’ shall refer to degradation in its ordinary sense as well as biodegradation, erosion, and dissolution.